126 OTHER FACTORS OF EVOLUTION 



vary together ; on the other hand it is impossible to imagine how 

 such correlated variations in different parts could spring up spon- 

 taneously and be developed by natural selection. 



Then, again, there is the case of the rudimentary hind limbs 

 of the whale, as to which Herbert Spencer says, " though during 

 those early stages of decrease in which the disused limbs were 

 external, natural selection probably had a share in decreasing 

 them, since they were then impediments to locomotion, yet when 

 they became internal, and especially when they had dwindled to 

 nothing but remnants of the femurs, it is impossible to suppose that 

 natural selection played any part : no whale could have survived 

 and initiated a more prosperous stirp in virtue of the economy 

 achieved by such a decrease." It is surely, as he says, much more 

 reasonable to attribute this extreme dwindling to the prolonged 

 effects of disuse perpetuated through innumerable generations. 



The effects of use and the exercise of a particular function 

 through many generations has been well shown by Prof. Brewer of 

 Yale University in his account of the " evolution of the trotting 

 horse." He says fast trotters were not wanted until the improve- 

 ment in roads and in wheeled vehicles during the last quarter of 

 the eighteenth century " caused an increasing demand for fast 

 roadsters for light draft." An authority on this subject, writing in 

 1796, considered that the utmost speed of an English trotter 

 (supposed to excel all others) to be a mile in two minutes and 

 fifty-seven seconds. During rather more than the next twenty 

 years, both in England and in America, a few of the best animals 

 " trotted a mile in three minutes," but none in less than the time 

 above given. After this period trotting "became a recognised 

 sport under specific rules," and "trotting records" began to be 

 regularly kept. The horses too were carefully bred, so that after 

 a time " heredity began to tell." By 1868 the speed had very 

 notably increased, so that "there were several horses in the 2:20 

 class, and nearly one hundred and fifty in the 2 130 list ; " while by 

 1892, " there were 5,908 in the 2:30 list, 507 in the 2:20 list, and 

 seven in the 2:10 list." ' 



We come next to a number of characters presented by man him- 

 self which seem to exemplify changes resulting from the inheritance 

 of functionally produced modifications. There is, for instance, the 

 varying powers of tactile discrimination possessed by the skin 



' Quoted by Cope in " The Primary Factors of Organic Evolution," p. 426, 



